1. Field of the Invention
The present invention relate to a device and a method for measuring data for calibration used when a lens aberration in a photographed image is corrected or when a lens aberration-corrected image for use in stereo image measurement or the like is necessary and, more particularly, to a device and a method for measuring data for calibration with which the internal parameters of a camera, such as the principal point position of the lens, screen distance (focal length) and distortion parameters, necessary to obtain a lens aberration corrected-image can be obtained with ease even if the camera is of the type in which optical conditions can be varied (a zoom camera, for example).
The present invention also relates to an image processing device capable of removing the effect of lens aberration in a stereo image photographed with a camera of the type in which optical conditions can be varied (a zoom camera, for example) to allow precise three-dimensional measurement of the shape of an object by stereo image measurement.
2. Description of the Related Art
Conventionally, it is important to obtain an image with low aberration in the fields of photogrammetry and photographic measurement. In the fields of photogrammetry and photographic measurement, high-accuracy lenses with a low aberration are therefore used. In the field of photogrammetry, the internal parameters of a camera (principal point position, screen distance, and distortion parameters) are analytically obtained in a way that a multiplicity of points measured precisely are positioned in a three-dimensional space and then measured from various angles. In the case of a measuring camera for use in the field of photographic measurement, the internal parameters of a camera are obtained by measuring precisely a fabricated camera.
However, an image photographed with a commercially available digital camera cannot be used in stereo image measurement of an object without correction because the lens distortion is large. Thus, it is necessary to correct the image using the internal parameters of the digital camera (principal point position, screen distance and lens distortion) and to measure the focal length of the digital camera at which the image was taken precisely to enhance the three-dimensional precision. Description will be hereinafter made of a fixed focus camera and a multi-focus camera separately.
Conventionally, camera calibration is performed on a fixed focus stereo camera and a photograph of an object for stereo image measurement is taken at its fixed focal point. However, a fixed focus stereo camera cannot adjust the focal length depending upon the objects, so that the image may be out of focus or the photographed object may not be large enough to fill the image area under some photographing conditions. Thus, it is difficult to obtain a stereo image of quality sufficient to be used in stereo image measurement. In addition, when the camera or the lens is changed depending upon the size of the object or the necessary precision to select a camera or a lens with a proper focal length, calibration must be performed again on the selected camera. Thus, three-dimensional measurement cannot be easily performed with a fixed focus stereo camera, which prevents spreading of stereo image measurement.
In a multi-focus digital camera employing a so-called zoom lens, the lens distortion is large, and the depth accuracy and the lens distortion vary with the focal length. Thus, when the focal length is changed, calibration must be performed again. It is, therefore, difficult to carry out image measurement using a zoom lens from the viewpoint of work efficiency. Thus, even a multi-focus camera is used as a fixed focus camera without using the function of the zoom lens, which makes stereo image measurement on site inconvenient.